Mass spectrometer



April 1 .1958 s.v N. FON-ER ET AL, y 2,829,259,

MASS SPECTROMETER RECORDER F/G. I

SAMUEL N. FONEI'? RICHARD L. HUDSON INVENTRS BY jfmww ATTORNEYS April l,1958 s. N. PONER ET AL l 2,829,259

MAss 'SPECTROMETER Filed Aug. 13, 1954' 2 Sheets-Sheet 2 T0 PHASESENSITIVE DETECTOR SAMUEL N. FONER RICHARD L. HUDSN 1N VENTORS BY @www-ATTRNEYS United States Patent O e 2,829,259 MAss sPECrRoMETER Samuel N.Feuer, Silver Spring, and Richard L. Hudson,

Rockville, Md., assignors to the United States of America as representedby the Secretary of the Navy Application August 13, 1954, Serial No.449,807

Claims. (Cl. Z50-41.9)

The present invention relates to improvements in mass spectrometers usedfor analyzing gases. In more detail, the invention relates to a massspectrometer particularly useful for detecting short lived intermediateswhich are formed in chain processes such as occur in the case ofcombustion reactions.

The detection of reaction intermediates, such as the atoms and radicalsformed in Chemical reactions, has long been a problem because of theshort life ofsuch intermediates and their low concentration levelswhencompared with the background signals generated in spectrometers. thepumps used to maintain low pressure levels in the gas sampling systemand the ion source are inefficient and do.

not remove all extraneous matter from the source. Thus, traces ofoxygen, nitrogen, and other elements are normally present in the ionsource, together with the radical or atom 'which it is desired todetect. When an oil diffusion pump is used to evacuate the source, theoil is also the origin of vcertain background signals. The pyrolysisproducts generated by a lament which is the source of electrons used toionize the gas molecules, also provide background noise. with the hotfilament are particularly annoying in the case of experiments withyatoms and radicals. A typical ex ample vis `carbon monoxide which isinvariablyformed by the reaction which takes place at the filament whenoxygen is in the system. Diftusely scattered and reacted molecules alsoseriously limit the useful sensitivity of prior art spectrometers.

The present invention hasl for its objectto provide a massspectrometer'in which the ratio of signal intensity to backgroundintensity is much greater thanthat of prior systems, thus makingpossible the detection of reaction intermediates.

A further object of the present invention is to provide a massspectrometer with a much higher effective sensitivity due to theimproved signal to noise ratio than has been attainable with priorspectrometers.

Still another object of the present invention is the provision of a massspectrometer having an'improved gas sampling system by which the gasmolecules can be quickly extracted from the reaction chamber andintroduced into the ion source free of collision with the walls of thesampling system. .t I

A more specific object of the invention is the provision 0f a massspectrometer including means for modulating` the molecular beam andemploying'phase detection ofthe ion current to improve the signal tonoiseratio.

Other objects and many of the attendant advantages of this inventionwill be appreciated readily as the same Fig; Zevis a view, partlydiagrammatic and partly seci In part, these background signals arisebecausel The products of reaction of the gas y electromechanicalmodulator.

2,829,259 Patented Apr. l, 1958 ICC tional, onanenlarged scale, showingthe details of one aspect of the system shown in Fig. l; and, Fig. 3 isa schematic diagram showing some of the details of the gas samplingsystem and ion source that are not shown in Fig. l. i Briey, the presentinvention comprises a gas sampling system by which molecules areextracted quickly and directed along a collision-free path to the ionsource. To achieve this, a gas sampling system comprising a plu-y ralityof successive orificed chambers is provided, the orifices of thechambers being in alignment. Diffusion pumps are individually providedto evacuate the different successive chambers of the sampling system.The molecular beam provided by this sampling system is coaxial with anelectron beam provided in the ion source, thereby increasing theprobability of ionizing the gas contained in the molecular beam. In onechamber of the gas sampling system, an electromechanical device' isprovided for periodically interrupting the molecular beam, therebysupplying a modulated molecular beam to the ion source. The ions formedby the interaction of the molecular beam and the bombarding electronsare passed through a conventional mass analyzer in which they aresubjected to a magnetic field controlled in magnitude and direction fordirecting the ions to a suitable detector. The ion current detectionsystem includes an electrometer amplifier and an electron multiplierwhich may be selectively connected to the mass analyzer to measure theion current and a phase sensitive detector which is synchronized with areference signal derived from the driving current of the Ion currentsignals which have random phase with respect to the modulator are`reship to the filament 41.

jected by this detecting system as noise. the phase sensitive detectoris then supplied to a suitable recorder, thus completing themassspectrometer of the present invention.

yIn more detail, referring to the drawings, it will be seen that the gassampling system 10 of the present invention includes a chamber 12 inkwhicha chemical reac tion takes place. The gases formed by the reactionare led in a beam from chamber 12 through a suitable sam-- pling orifice14 intoa chamber 16. A mechanical modulator or chopper 18 is positionedto interrupt the molecular beam led from chamber 12, at a frequencywhich is` preferably the resonant frequency of the chopper itself;

The chopper is actuated by a suitable chopper driver 20 to whichoscillatory currents are supplied by an amplitude l.

stabilized oscillator 22. As will be described later, a

pickup may be positioned in chamber 16 for detecting theI vibrations ofthe chopper 18 and providing a feedback signal utilized by theoscillator.

pleting the gas sampling system. As can be seen in Fig. 3, the` chambers16 and 28 are provided with connections 34 for attachingl diffusionpumps, not shown, which evacuate the chambers.

rl`he modulated `molecular beam is thus conducted through the slit 30into chamber 3-6 which houses an ion source generally indicated byreference numeral 38.

ly to Fig. 3, a filament 41 serves as a source of electrons,l and anaccelerating grid 42 is provided adjacent the source. For controllingthe electrons, an auxiliary grid 43 is arranged at each end of thechamber 40 andl an electron collector 44 is provided in spaced relation?Of course, suitable potentials '-'f n are applied to the grids and thecollector, and in addi@ The output of tion,aimagneticitield from asourcenot'shown is provided for collimating the electrons from the filament41. The

molecular beam is coaxial with the electron beam formed in -the`ion-source thereby increasing the probability "of ionizing the incominggas molecules. -T he `ions formed inthe chambrl40 are expelledby atransverse electric field throu`gh`aslit'50 formed inthe wall ofsaidchamber and into `the tube 52 which is a part of the mass analyzergenallyindicated by `reference `character 54.

Asisconventionah an electromagnet 55 is arranged adjacent ito the tube52 forproviding a magnetic eld to control "the path ofthe ions emittedVfrom `the ion source v38. The ions Vpassing through the tube 52arefocusedona 'slit 56 by the magnetic field of the electromagnet 55 *8nd,by varying the strength of the field, ions of different mas'sf'can' becaused `to enter the chamber 581whichhou`sesmeanstocollect the ions orpass them `on 'to `a suitable detector. A gate `69 is pivotally mountedVin the chamber158 'and may bemoved from a position `shown in dottedlines to full line position by an Yexternal operating mechanism (notshown). When in the ldotted line position, "the gateA conducts the ioncurrent tothe inputterminals-ofan electrometer amplierh64 4ofconventional t`y'pe. `On the `other hand, when the gate60 is in the fulllinepositiom the ions will pass between electrodes 65 andAA66,"th`rough"opening 68, and into yan electron multiplier `detector70, which is also of conventional structurep `Whichever is selected, theelectrometer amplifier 64 or the electron multiplier detector 70,'can`be connected by `a two position switch 72 "to a phase sensitivedetector74. Either of these devices 64 or detects and `arripliies theion current flowing through`the-slot 56 of the mass analyzer 54. Theelectrometer `amplifier is particularly useful when it is desired tosupply an :amplifier signal representing ion current to a recorder branoscilloscope for immediate visual examination. The electron multiplierdetector is particularly suited in other applications, as for example,where it is desired to use an ion counter or similar device.

The 'swtch`72 will conduct the signal representing the detected ioncurrent to a phase sensitive detector 74. A reference `signal'for thephase sensitive detector is supplied from`the `oscillator 22 'whichprovides the driving current `to the chopper driver`20. The phasesensitive detector 74 detects uonlyithat partof thesignal representin'g`iou curret Which is in phase with the vibrations of the 'choppermodulator `8; The output of thedete'ctor 74 isfedto'a recorder 78.

The :details ofthe chopper and the associated equip ment for modulatingthe molecular beam are shown in Fig. 2. The chamber 1'6 of the gassampling system is shown as being generally rectangular in cross`section and having an opening 79 in its top wall. A parot aperturedlugs 80 and 82 lp'rovided on the opposite side walls of the :chamber 16slidably receive `posts 84 and 86,uv respectively. `At their upper endsthe posts 84 and 86 supporta plate 88 which carries the chopper 18 and adriving mechanism generally indicated by reference numeral 90.` Theplate V88 is provided with slots for receiving the upper ends of theposts 84 and 86, and screws I 92 and 94 threaded in apertures providedina `rim 96 on the plate 88 permit lateral adjustment of the chopper 18`so thatthe latter interrupts the molecular beam in such amanner as toprovide a symmetrical waveform when the ioncurrent'subsequently formedis detected.

A bellows surrounds the opening 79 provided in the topwall of chamber`16, the lower end of the bellows being attached to said top wall andthe upper end there of being attached to the, underside of thesupporting plate The chopper 18 is inthe nature of a vibrating reed type`of apparatusand includes an elongated, partially` hollow section 104which terminates at the lower end in a broad `portion -106 positioned toperiodically in- `terrupt the molecular beam passing through orilice 26.

The` hollow section 104 receives, in its upper end, the lower end 113 ofa rod-shaped element 114 which is made of suitable magnetic materialsuch as soft iron, said lower end 113 being machined to t snugly insidethe hollow section 104. The periphery of the rod-shaped section 114 ofthe chopper 18 is notched as shown at 111 and 112 so that the chopper 18vibrates about this point as a node. At its upper end, the chopper 18has an enlarged section 108 which is received snugly in a sleeve memberintegral with the suporting plate 8S so that the chamber 16 is sealed.

The chopper driving mechanism 90 actuates the chopper 18, preferably atAits fundamental frequency. The driver includes a coil which partiallyills the space provided internal to a permanent magnet yoke 122,suitably polarized as indicated. The yoke 122 is provided with apertures123 and 123 which snugly receive the enlarged section 108 of the chopper18 and the sleeve 110 formed on plate 88, respectively. Potting compoundindicated by reference numerals 124 and 126 iills the space between thecoil and the permanent magnet 122, and the space between said permanentmagnet and an external shell 128, respectively. An oscillator is coupledby a transformer 132 to the coil 120, to supply operating currentthereto.

To provide the required phase relationship between the vibrations of thechopper and the oscillatory currents used to drive the chopper, it maybe desirable to employ a feedback arrangement including a capacity typepickup 134 `positioned adjacent the chopper and a preamplifier 135inserted between the pickup and the oscillator 130. The capacity pickup134 includes a plate 138 carried at the lower end of a rod 1.40 ofconducting material, the rod being supported in an insulating seal 142in the plate 88. An electrical lead 144 having a coupling condenser 145inserted therein interconnects the plate 138 and the` grid 146 of atriode vacuum tube 148, which isconnected as a cathode follower. Asource of positive voltage is connected directly to the plate of tube148 and through a current limiting resistor 152 to the plate 138 of thecapacity pickup 134. A blocking capacitor 154 is connected between thesource of positive voltage and ground, and an electrical lead 156connected to the plate 88 completes the interconnection of the capacitypickup and the preamplifier 135.

For providing a suitable bias, the grid 146 is connected to the junctionof resistors 158 and 160 which comprise a voltage divider connectedbetween the source of` vpositivevoltage and ground. Bias for the cathode162 is provided by resistor 164 and an. output connection 166 is Imadeto the. cathode 162. With the choice of the proper circuit components,the output of the preamplier 135 is fed to the oscillator 138 in theproper phase for maintaining oscillations. The oscillations thusgenerated are coupled by the transformer 132 to the chopper driver 90,causing the chopper to vibrate. A second transformer 170 is used toprovide a reference signal for the phase sensitive detector 74.

In operation, a chemical reaction is initiated in the chamber 12.Typical reactions are the burning of hydrogen in oxygen, and methane inoxygen. A sample of the gas generated by the reaction flows through theorifice 14. The molecular beam thus formed is modu lated by themechanical chopper 18 at a frequency determined by the characteristicsof the chopper and the system for driving the chopper. The centralportion of the modulated beam passes through the orice 26 into chamber28 and thence through orifice 30 into the chamber 40 in the ion source38. In the ion chamber, ions result from the interaction of a beam ofelectrons from the filament 41 and the molecules entering the chamber,the electron beam being formed and controlled by a collimating magneticfield and an electrostatic eld in a manner well known in the art. Atransverse electric iield provided in the ion source expels the ionsfrom the source into a conventional mass analyzer 54. By varying thestrength of the magnetic teld associated with the analyzer, ions ofdifferent mass can be caused to pass through the slit S6 into chamber58. The gate 60 permits the selection of either the electrometeramplifier 64 or the electron multiplier detector 70 for detecting thetotal ion current passing through the slit S6. The iselected one ofthese devices is connected through a switch 72 to the phase sensitivedetector, thereby supplying a signal including7 background and the ioncurrent which it is desired to measure'to the phase sensitive detector.The reference signal for the phase sensitive detector is provided by theoscillator employed to drive the chopper. The phase relationships areadjusted so that the background is rejected, only that part of theunknown signal due to the ion current which is in phase with thevibrations oi' the chopper being detected.

It is clear therefore that the signal to noise ratio is much greater inthe present mass lspectrometer than in prior spectrometers. Thebackground due to reacted molecules, molecules that have collided withthe walls of the chamber, and other sources discussed above, issubstantially eliminated by this invention.

Experiments have been conducted to determine the degree of backgrounddiscrimination actually realized by this technique. An argon beam fromthe llame chamber which gave a detector signal of three millivolts,corresponding to about 400 ions per second, was observed while abackground pressure of argon was developed in the ion source byadmitting argon directly. The modulated beam could be detected in thepresence of a 40 volt D. C. background signal so that a discriminationfactor of approximately 104 was obtained. In the case where thebackground arises from scattered beam molecules, including decompositionproducts formed at the filament, the collimation of the molecular beamintroduces an additional discrimination estimated to be a factor of 10because the molecular density in the beam s about 10 times the scatteredmolecular density in the apparatus. Other experiments were performedwith gases such as hydrogen and oxygen which are known to react on thefilament to produce hydrogen atoms and carbon monoxide, respectively, tosee if pyrolysis products would be observed with the chopped beammethod. No indication of decomposition products was observed.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

l. A mass `spectrometer for analyzing a gas, comprising means forforming a beam of molecules of the gas, means for modulatingsaidmolecular beam, means for forming ions from said molecular beam, a massanalyzer through which the ions thus formed are passed, and means fordetecting that part of the ion current passed through said analyzerwhich is snychronized with the modulation of said molecular beam.

2. A mass spectrometer for detecting atoms and radicals formed inchemical reactions, comprising a reaction chamber, means for extractinga beam of molecules of the gas formed in the reaction chamber from saidchamber, means adjacent the reaction chamber for modulating saidmolecular beam, means for forming ions from the modulated molecularbeam, a mass analyzer `through which the ions thus formed are passed,and means for detecting that part of the ion current passed through theanalyzer which is synchronized with the modulation of said molecularbeam.

3. A mass spectrometer for detecting atoms and radicals formed in achemical reaction, comprising a reaction chamber having a samplingoriiice in one wall thereof, said orifice emitting a beam 'of moleculesof the gas formed in said chamber, means adjacent said reaction chamberfor periodically varying the concentration lof molecules in said beam,means to which said modulated beam is supplied for forming ions fromsaid beam, a mass analyzer through which said ions are passed, and meansfor detecting that part of the ion current passed through said analyzerwhich is in synchronization with the periodic variations ofconcentration of said beam.

4. A mass spectrometer for detecting atoms and radicals formed in achemical reaction, comprising a reaction chamber, said reaction chamberhaving an orifice for emitting a collimated beam of molecules of the gasin the reaction chamber, means positioned adjacent said reaction chamberfor periodically interrupting said molecular beam, means for energizingsaid last named means, means for forming ions from said molecular beam,a mass analyzer through which said ions are passed, and a phasesensitive detector to which a signal representing the ion current passedthrough said mass analyzer is supplied, the reference signal for saidphase sensitive detector being in phase with the periodic interruptionsof said molecular beam, whereby only that part of the ion current whichis synchronized with said interruptions is detected.

5. A mass spectrometer for detecting atoms and radicals in a chemicalreaction, comprising a reaction chamber having an orice for emitting acollimated beam of molecules of the gas formed in said chamber, meanspositioned adjacent said chamber for periodically interrupting said beamof molecules, an amplitude stabilized oscillator for energizing saidinterrupting means, means for forming ions from the molecular beam, amass analyzer through which said ions are passed, means for detectingthe ion current passed through said analyzer, and a phase sensitivedetector to which a signal representing the ion current passedthroughsaid analyzer is fed, the reference signal for said phase sensitivedetector vbeing supplied from the amplitude stabilized oscillatorwhereby only that part of the ion current which is in synchronism withthe interruptions of said molecular beam is detected.

References Cited in the tile of this patent UNITED STATES PATENTS2,221,467 Bleakney Nov. 12, 1940 2,457,162 Langmuir Dec. 28, 19482,694,151 Berry Nov. 9. 1954

